Carbon-13 Relaxation Time Measurements in Formic Acid

Abstract

The variations in carbon-13 nuclear magnetization with time have been determined for formic acid under proton-coupled and proton-decoupled conditions by the adiabatic rapid passage technique. The time-dependent variations for proton-coupled carbon-13 magnetization are nonexponential, while the nonequilibrium, proton-decoupled carbon-13 magnetization follows an exponential relationship. By applying the theoretical equations to the experimental results, spin–lattice relaxation times for the spin-decoupled case were calculated for both the formyl proton and carbon-13 nuclei as well as for the internuclear process. It is demonstrated that the carbon-13 relaxation process is dominated by the mutual relaxation of formyl protons and carbonyl carbons through the intramolecular dipole–dipole mechanism. Various means of extracting relaxation data with and without spin-decoupling equipment are discussed for the four-line two-spin system.